1. Field of the Invention
The present invention relates generally to improvements in magnetic recording methods and apparatus, and, more particularly, but not by way of limitation to improvements in recording methods an apparatus for hard disc drives.
2. Brief Description of the Prior Art
In a hard disc drive, data generated by a computer is stored on concentric tracks defined in a magnetizable surface coating on a rotating disk by a read/write head that "flies" over the surface coating in close proximity to the disc surface. To this end, the drive is comprised of a write amplifier which receives the data in the form of a serial stream of data bits that are each expressed as one of two logic levels and responds by passing a current through the read/write head that magnetizes lengths of the track in on of two directions. More particularly, the direction of the current is reversed each time a bit having a non-zero logic level is received to reverse the direction of magnetization of the track. Such a magnetization reversal can be viewed as a "magnetic charge" at the location at which the magnetization reversal occurs with the sign of the "charge" being determined by the directions of the magnetization of the track to either side of the reversal. The rotation of the disc is maintained substantially constant and the data stream is clocked into the write amplifier at a constant rate so that the spacing of these "charges" along the track reflects the temporal spacing of the non-zero bits in the data stream. As will be clear to those skilled in the art, these "magnetic charges"; that is, the magnetization reversals, will produce a total magnetic field adjacent the disc that is a superposition of the fields produced by the individual magnetization reversals and which rotates with the disc.
When the data is read back, the read/write head is used as a pickup coil that responds to changes in the total magnetic field in which it is immersed to generate an electromotive force. For widely spaced magnetization reversals, only the magnetization reversal nearest the read/write head contributes significantly to the magnetic field at the read/write head with the result that the emf has the form of a series of emf pulses, each associated with only one of the magnetization reversals and each having a peak value that occurs when the read/write head is aligned with the portion of the track at which the magnetization reversal occurs. Accordingly, the emf pulse peaks have a temporal spacing that reflects the spacing of the magnetization reversals along the track and the emf pulses can be peak detected to provide a stream of digital pulses having an edge that corresponds to the peaks of the emf pulses. Since the spacing of the magnetic reversals reflects the temporal spacing of the non-zero bits of the original data stream and the temporal spacing of the emf peaks reflects the spacing of the magnetic reversals along the track, the data that is read from the disc is recovered in the form of a data stream that, ideally, is identical to the data stream that was written to the disc.
In practice, several effects interfere with this identity of the written and read data streams. Thus, for example, the rotation rate of the disc varies with time so that the temporal spacing of the emf peaks is not the same as the temporal spacing of the non-zero bits of the data stream that is written to the disc. This effect is commonly overcome by passing the pulses generated in the read circuit through a phase locked oscillator whose output defines a series of "read windows" that are locked to the stream of bits returned from the disc.
A second effect, an effect which not only causes a departure of the written and read data streams from an identity but can also interfere with the phase locking of the read circuit on the returned data stream, is so-called "peak shift" that arises when the magnetic reversals are closely spaced along a data track. When the magnetic reversals are closely spaced, the magnetic fields produced at the read/write head by magnetic reversals other than the magnetic reversal nearest the head will contribute to the total magnetic field in which the read/write head is immersed sufficiently to cause a temporal shift in the emf peak associated with the nearest magnetization reversal. Thus, while the emf induced in the head will still have the form of a series of pulses generally associated with individual magnetization reversals, the temporal spacing of the peaks of the pulses will no longer reflect the spacing of the magnetization reversals along the track. Peak shift causes shifting of the read windows so that the digital pulses generated in the read circuit in response to the emf peaks can occur in the wrong read windows with the result that the data that is recovered from the disc is not the same as the data that was written to the disc.
A commonly adopted solution to peak shift is prewrite compensation of the data that is written to the disc. In prewrite compensation, each non-zero bit of the data stream that is written to the disc is temporally shifted, in relation to previous and ensuing bits, as the data stream is written so that the pattern of magnetization reversals along a data track is distorted by an amount that is just sufficient to cause the temporal spacing of the emf peaks induced in the read/write head, upon reading of the data, to be very nearly identical to the temporal spacing of non-zero bits in the original data stream.
In disc drives that have limited storage capacity so that the magnetization reversals have a relatively large spacing, prewrite compensation can be effected with simple circuitry that can be easily included on the surfaces of silicon chips on which other circuits of the disc drive are formed using large scale integration techniques. Thus, for example, in U.S. Pat. No. 4,633,336, issued Dec. 30, 1986 to Horie, et al., prewrite compensation is carried out by passing the data stream to be written to the disc through a series of three flip flops and transmitting each non-zero bit that appears in the central flip flop to the write circuit with, or without, a delay depending upon the logic levels in the remaining two flip flops. Thus, the prewrite compensation circuit taught by Horie et al. provides compensation only with respect to the logic levels of the two bits that are nearest in the data stream to a bit that is written.
In disc drives that have a larger storage capacity, attained in part by writing the data stream to be stored at a high frequency that will result in close spacing of the magnetization reversals along a data track, prewrite compensation becomes more difficult. Because of the close spacing of the magnetization reversals, several non-zero bits preceding and following the bit that is currently being written can have an effect on the temporal spacing of the peaks of the emf pulses induced in the read/write head when data is recovered from the disc. Thus, the prewrite compensation requires more complex circuitry. One technique that has been used is to store appropriate amounts of delay in a ROM and address the ROM with the pattern of data bits about the bit to be written as taught in U.S. Pat. No. 4,000,513, issued Dec. 28, 1976 to Precourt.
The problem of circuit complexity in effecting prewrite compensation has been exacerbated by recent developments in the disc drive art designed to maximize the data storage capacity of disc drives. As disclosed in U.S. Pat. No. 4,799,112, issued Jan. 17, 1989 to Bremmer et al., the teachings of which are hereby incorporated by reference, the storage capacity of a disc drive can be maximized by dividing the surfaces of the discs into zones and writing data to each track in a zone at a zone frequency that is selected to optimize resolution of the data stream that is read from the discs. Because of the writing of data at a plurality of frequencies, each of which can require different amounts of prewrite compensation, circuitry for effecting prewrite compensation, if it is to be effective, can become very complex indeed.
This complexity introduces new problems. Since compensation is effected as a stream of data bits is written to the disc, the appropriate amount of delay of the non-zero bits must be determined and applied very rapidly so that time is generally not available, without limitations on the write frequency, to recover the appropriate amount of delay from a memory device that is formed on a chip that is separate from remaining portions of the prewrite compensation circuit. While a suitable memory device can be formed on the chip that contains remaining portions of the prewrite compensation circuit using large scale integration techniques a price is exacted for doing so. The more complex a large scale integrated circuit becomes, the more silicon surface area the circuit occupies and the more expensive the integrated circuit becomes. Thus, until the present invention, the manufacturer of disc drives has been faced with a Hobson's choice. He can limit the frequencies at which data is written to discs, and thereby limit the data storage capacity of his drives, or he can increase his manufacturing costs to provide his drives with suitable prewrite compensation circuitry.